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Saurolophus Angustirostris (Dinosauria: Hadrosauridae), from the Upper Cretaceous of Mongolia

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Saurolophus Angustirostris (Dinosauria: Hadrosauridae), from the Upper Cretaceous of Mongolia RESEARCH ARTICLE Perinatal Specimens of Saurolophus angustirostris (Dinosauria: Hadrosauridae), from the Upper Cretaceous of Mongolia Leonard Dewaele1,2*, Khishigjav Tsogtbaatar3, Rinchen Barsbold3, Géraldine Garcia4, Koen Stein5, François Escuillié6, Pascal Godefroit1 1 Directorate 'Earth and History of Life', Royal Belgian Institute of Natural Sciences, rue Vautier 29, B-1000, Brussels, Belgium, 2 Research Unit Palaeontology, Department Geology and Soil Sciences, Ghent University, Krijgslaan 281, 9000, Ghent, Belgium, 3 Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, 210–351, Mongolia, 4 Université de Poitiers, IPHEP, UMR CNRS 7262, 6 rue M. Brunet, 86073, Poitiers cedex 9, France, 5 Earth System Science, AMGC, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium, 6 Eldonia, 9 Avenue des Portes Occitanes, 3800, Gannat, France * [email protected] OPEN ACCESS Abstract Citation: Dewaele L, Tsogtbaatar K, Barsbold R, Garcia G, Stein K, Escuillié F, et al. (2015) Perinatal Specimens of Saurolophus angustirostris Background (Dinosauria: Hadrosauridae), from the Upper The Late Cretaceous Nemegt Formation, Gobi Desert, Mongolia has already yielded abun- Cretaceous of Mongolia. PLoS ONE 10(10): dant and complete skeletons of the hadrosaur Saurolophus angustirostris, from half-grown e0138806. doi:10.1371/journal.pone.0138806 to adult individuals. Editor: Andrew A. Farke, Raymond M. Alf Museum of Paleontology, UNITED STATES Received: April 22, 2015 Methodology/Principal Findings Accepted: September 3, 2015 Herein we describe perinatal specimens of Saurolophus angustirostris, associated with fragmentary eggshell fragments. The skull length of these babies is around 5% that of the Published: October 14, 2015 largest known S. angustirostris specimens, so these specimens document the earliest Copyright: © 2015 Dewaele et al. This is an open development stages of this giant hadrosaur and bridge a large hiatus in our knowledge of access article distributed under the terms of the Creative Commons Attribution License, which permits the ontogeny of S. angustirostris. unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Conclusions/Significance Data Availability Statement: All relevant data are The studied specimens are likely part of a nest originally located on a riverbank point bar. within the paper and its Supporting Information files. The perinatal specimens were buried by sediment carried by the river current presumably Funding: The author(s) received no specific funding during the wet summer season. Perinatal bones already displayed diagnostic characters for for this work. Eldonia provided support in the form of Saurolophus angustirostris, including premaxillae with a strongly reflected oral margin and a salary for FE, but did not have any additional role in upturned premaxillary body in lateral aspect. The absence of a supracranial crest and the study design, data collection and analysis, unfused halves of the cervical neural arches characterize the earliest stages in the ontog- decision to publish, or preparation of the manuscript. The specific role of this author is articulated in the eny of S. angustirostris. The eggshell fragments associated with the perinatal individuals ‘author contributions’ section. can be referred to the Spheroolithus oogenus and closely resemble those found in older for- Competing Interests: François Escuillié is employed mations (e.g. Barun Goyot Fm in Mongolia) or associated with more basal hadrosauroids by Eldonia a company specializing in the preparation (Bactrosaurus-Gilmoreosaurus in the Iren Dabasu Fm, Inner Mongolia, China). This PLOS ONE | DOI:10.1371/journal.pone.0138806 October 14, 2015 1/33 Saurolophus Babies from Mongolia and casting of fossils. There are no patents, products observation suggests that the egg microstructure was similar in basal hadrosauroids and in development or marketed products to declare. This more advanced saurolophines. does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors. Competing Interests Abbreviations: MPC, Institute of Paleontology and One of the authors (FE) is employed by the commercial organization Eldonia. Eldonia pro- Geology, Mongolian Academy of Sciences, Ulaan vided support in the form of a salary for FE, but did not have any additional role or influence Baatar, Mongolia; PIN, Paleontologiceski Institut, Academii Nauk, Moscow, Russia; RBINS, Royal in the study design, data collection and analysis, decision to publish, or preparation of the Belgian Institute of Natural Sciences, Brussels, manuscript and it does not alter the authors’ adherence to all the PLoS ONE policies on Belgium; ZPAL, Institute of Paleobiology of the Polish sharing data and materials. Academy of Sciences, Warsaw, Poland. Introduction The 'Dragon’s Tomb' dinosaur locality was discovered in 1947, in the Nemegt Formation (late Campanian / early Maastrichtian, Late Cretaceous) of the Gobi Desert, by the Russian Palaeon- tological Expedition to Mongolia’s Gobi Desert, led by I. A. Efremov. The bone bed at this site has yielded numerous articulated skeletons of the giant saurolophine hadrosaurid Saurolophus angustirostris Rozhdestvensky, 1952 [1]. This dinosaur is particularly abundant in the whole Nemegt Formation, comprising approximately 20% of all vertebrate fossils [2] found. The skull lengths of the known S. angustirostris specimens extend from about 437 mm (MgD-1/159) up to 1220 mm (PIN 551/357), thus already covering a wide array of ontogenetic stages, from juveniles to adult individuals. However, embryonic and neonatal remains have not been described to date. Because of the quantity and quality of dinosaur skeletons from Mongolia, the whole of the Nemegt Formation has become a favorite target for poachers, and untold numbers of Saurolo- phus specimens are now in private hands around the world or have been destroyed in the pro- cess of poaching. Here, we describe and discuss specimen MPC-D100/764: an exceptional block of perinatal specimens of Saurolophus angustirostris, with associated eggshell fragments, from the Nemegt Formation. The skull length of these babies is around 5% of that of the largest known S. angu- stirostris specimens, so they document the earliest development stages of this giant hadrosaur and bridge a large gap in our knowledge of the ontogeny of Saurolophus angustirostris. Materials and Methods Originally poached from the Nemegt Formation, specimen MPC-D100/764 resided in a private collection for an unknown amount of time. Neither the exact geographic nor stratigraphic ori- gin is known. The specimen has been treated chemically in order to solidify the matrix. This is evidenced by both stains on the specimen and a differential mechanical resistance between the outer surface and the interior. In 2013, the specimen was transferred to the Royal Belgian Insti- tute of Natural Sciences (RBINS), Brussels, through the French company Eldonia. Subsequent negotiations between the RBINS, Eldonia and Mongolian authorities led to the official return of the specimen to Mongolia, where it is now catalogued as specimen MPC-D100/764. One of the femora and one dorsal vertebral centrum with associated neural arch were sam- pled for osteohistological analysis. The femur was sectioned at the Research Unit Mineralogy and Petrology, Department Geology and Soil Sciences at Ghent University, Belgium, whereas the vertebral elements were sectioned in the Service de Paléontologie Animale et Humaine, PLOS ONE | DOI:10.1371/journal.pone.0138806 October 14, 2015 2/33 Saurolophus Babies from Mongolia Departement of Geology at the University of Liège, Belgium. The longitudinal section of the femur was machine-ground to a thickness of 30 μm, the vertebral sections were ground to 50 μm. Sections were studied using a polarized light microscope Olympus BH-2 (femur) and a Nikon LV 100 (vertebra) polarized light microscope. Pictures of the thin sections were taken with a ColorView I (femur) and a QImaging MP5.0 (vertebra) digital microscope camera. The femur was selected because its poor state of preservation prevented the accurate description of the bone and because it could easily be removed from the block without damaging any other elements. However, because of its poor state of preservation and post-mortem displacement, the exact orientation of the longitudinal thin section cannot be elucidated. The dorsal centrum was selected because it was easily accessible, and would allow comparison with other published dorsal vertebrae of embryonic dinosaurs. During further preparation of MPC-D100/764 in Belgium, two fragmentary eggshells were found closely associated with the skeletal material, suggesting that the individuals were still enclosed in their eggs when they were covered by sediments, or that they died shortly after hatching. The description of the outer and inner surfaces of the eggshell fragments is based on macroscopic examination and by using a binocular microscope, while observation of their crystallographic ultrastructure was made by scanning electron microscopy (SEM). Because there are only two small eggshell fragments associated with MPC-D100/764, the study of the eggshell microstructure through thin sections, requiring a large number of sections in three dimensions has not been attempted: this process would have destroyed the eggshell fragments for future research. Nomenclature and
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